The invention relates to an output amplifier for a transmitting tube, more specifically for a transmitting tetrode having an operating frequency located in the frequency band from 40-400 MHz. The output amplifier includes a first cylindrical conductor provided at one end with contacts to which the electrodes of the transmitting tube can be connected and a second cylindrical conductor which is arranged coaxially relative to the first cylindrical conductor. The two conductors form part of a first impedance transformer for transforming the input impedance of the transmitting tube to a first input impedance at the other end of the coaxial conductors.
Output amplifiers of this type are generally known and are used in, for example, FM radio and television transmitters. The output amplifier is provided to amplify the applied input signal which has been preamplified to for example, some hundreds of Watts, to the ultimate transmitting power of, for example, 30 kW. In order to match the output impedance of the preamplifier (for example 50 Ohms) to the input impedance of the active portion of the transmitting tube (for example 12 Ohms in parallel with 62 pF) the prior art output amplifiers utilize impedance transformers in the form of quarter-wavelength coaxial lines. In order to achieve a reasonably broadband impedance match, the impedance transformation is realized in a number of steps. Conventional output amplifiers are amplifiers, inter alia with two series-arranged, quarter-wavelength, coaxial conductors arranged in air with which the desired input impedance of the output amplifier is obtained. Such output amplifiers are comparatively bulky, as in the VHF-range at 200 MHz .lambda./4 is approximately 37.5 cm so that the two required transformers have an overall height of 75 cm. In band 2 of the VHF range, commonly used for FM radio transmission (88-108 MHz) the overall height is even of the order of 150 cm. In addition, the output amplifiers comprise means for tuning the input of the output amplifier to the operating frequency within the frequency range band 2 and 3 of the VHF range for which the output amplifier is suitable.